Incorrect CT Phase Angles leading to wrong power readings
Incorrect CT Phase Angles leading to wrong power readings
(OP)
We are bringing a new plant power distribution system online which is the reason for alot of my posts lately. I am a fairly new engineer who is trying to learn a lot of this stuff as it happens. I appreciate all of the help from this forumn.
On our main breaker we have a Multilin 750 protective relay, and a Multilin PQMII power quality meter. When looking at the currents on the Multilin, they are showing up strange for they are blinking between some current value and zero. I'm not sure if this is because I have a bad relay, or because of what I am seeing on my PQM meter which is as follows:
When looking at the PQMII meter I notice that I am getting negative kW power readings. I find this strange, so when I look at the current phasors they look strange as well. I know this negative kW reading is incorrect because I have another PQMII meter downstream measuring the exacty same load giving the exact same readings except the numbers are positive. This is why I am thinking that I may have a CT or a PT out of phase on the PQMII and ML 750 on the main breaker, which are both looking at the same CT's. Here are the readings I took off of the PQMII.
Voltge Phasors:
Vab = 4137 @ 0 deg LAG
Vbc = 4181 @ 120 deg LAG
Vca = 4172 @ 300 deg LAG
Current Phasors:
Ia = 27A @ 255 deg LAG
Ib = 27A @ 18 deg LAG
Ic = 27A @ 130 deg LAG
Three phase real power = -152 kW
Three phase Reactive Power = -130 kVAR
Three phase Apparant Power = 195 kVA
Three phase power factor = .74 LAG
I'm guessing that the CT's are out of phase with the PT's. Does this make sense?






RE: Incorrect CT Phase Angles leading to wrong power readings
Get the instruction manual and look at the wiring diagram VERY closely, paying particular attention to the power flow sign convention and the CT polarities shown. If you are using doughnut CTs, make sure they are all mounted in the same orientation. It makes a difference if you flip one.
The PQM may be able to show you a phasor diagram, which can be helpful in troubleshooting connections.
RE: Incorrect CT Phase Angles leading to wrong power readings
RE: Incorrect CT Phase Angles leading to wrong power readings
RE: Incorrect CT Phase Angles leading to wrong power readings
RE: Incorrect CT Phase Angles leading to wrong power readings
When looking at this problem I have been getting hung up with the representation of the different voltage and current phasors. I have drawn the two phasor diagram scenarios I've been confused between in the attached document. Is the correct way for representing the voltage phasor diagram shown in Case I, or Case II of the attachment.
Which diagram would depict the phasors that I have listed above? I am thinking that it is the left diagram in Case II. Is this correct? When looking at these diagrams which diagram is considered positive sequence and which is considered negative sequence rotation?
When looking at a phasor such as Vbc, is this measuring the angle of b with respect to c which in this case is lagging?
(I know the phasors above have an angle of 300deg however for my diagrams I am using an hypothetical 240deg)
RE: Incorrect CT Phase Angles leading to wrong power readings
Sorry forgot the attachment!
RE: Incorrect CT Phase Angles leading to wrong power readings
If you get the meter hooked up correctly, it should work.
You just need to draw out the connection diagram including all CT and PT polarity dots to verify that you have everything connected per the meter's wiring diagram.
RE: Incorrect CT Phase Angles leading to wrong power readings
In this diagram, if you have the whole lot rotating anti-clockwise, the A phase will pass through the 0 degrees point first, then followed by the B phase - 120 degrees later, then followed by pahse C, lagging phase A by 240 degrees and phase B by 120 degrees.
This is the standard convention. If you looked at the three phases on an oscdilloscope this will give a A-B-C across the screen.
RE: Incorrect CT Phase Angles leading to wrong power readings
See attachment.
RE: Incorrect CT Phase Angles leading to wrong power readings
Sibeen
Thanks for the picture. Your right a picture is worth 1000 words, I now understand the concept of counter clockwise rotation.
The only part I have left to question is labeling the phase angles. Ignoring the voltage vectors for a minute when you are labeling the polar coordinates for the vector representation do you start with the positive x-axis and lebel the degrees clocwise or counter clocwise? In other words which of the following cases below represents the proper phase angles for your drawing assuming the vector magnitude is 120?
Rotation is ABC
Case I: A phasor = 120@ 0deg
B phasor = 120@ 240deg (-120deg)
C phasor = 120@ 120deg (-240deg)
or
Case II: A phasor = 120@ 0deg
B phasor = 120@ 120deg (-240deg)
C phasor = 120@ 240deg (-120deg)
RE: Incorrect CT Phase Angles leading to wrong power readings
RE: Incorrect CT Phase Angles leading to wrong power readings
Ok Thanks I see it now!
So with the phasors in my origonal post, the phase rotation is ACB. The only issue here is the 300deg on phase C instead of 240deg (possibly a PT issue)
The thing that I have to investigate more is that on my Multilin 750 relay there is a phase sequence monitor that indicates a positive sequence voltage for my voltage phasors. Looking at the relay program the phase sequence in the relay is set up for ABC. I would assume the realy would indicate a positive sequence if my phase rotation was indeed ABC, however since my phase sequence is actually ACB then I would expect the realy to read a negative sequence phase rotation.
I'm going to go re-check this setting, but does anyone know why I'm getting this positive sequence indication?
RE: Incorrect CT Phase Angles leading to wrong power readings
about 300deg "problem".
Please pay attention again on the David's post.
300deg of Vca it's 180deg from the Vbc, that mean your wiring is Vcb.
Firstly, please check it with simple Fluke on the terminals of your device..
Regards.
Slava
RE: Incorrect CT Phase Angles leading to wrong power readings
Slava
What should I be looking for with a fluke meter when checking these terminations on the device?
How do you properlay state the term Vca?
Does this read the voltage at C with respect to A?
How about Vbc? Does this read the voltage at B with respect to C?
RE: Incorrect CT Phase Angles leading to wrong power readings
Unless you have strong understanding of voltage and current phasors and the conventions used in defining them, my suggestion is to trace out the PT and CT wiring very carefully, verifying all polarities and phases. Then compare this to the meter wiring diagram provided by the manufacturer.
I have seen hours and hours wasted by trying to guess where the problem was, swapping wires, changing polarities and phases, etc and ending up totally flummoxed.
OK, back in my hole...
Dave
RE: Incorrect CT Phase Angles leading to wrong power readings
It's very old and very simple voltage wiring test.
Assume voltages terminal of your device are:
1 Va
2 Vb
3 Vb
4 Vc
5 Vc
6 Va
Now with Fluke you chek voltage between:
1-2 110V
1-3 110V
1-4 110V
1-5 110V
1-6 0V
2-3 0V
2-4 110V
2-5 110V
2-6 110V
I assume you see 2-6 "0V", that mean some mistake in wiring.
Regards.
Good Luck
Slava
And after this you can check rotation setting
RE: Incorrect CT Phase Angles leading to wrong power readings
The PT's are configured in an Open Delta Connection. On the secondary side Vb input to the relay is gounded and also jumpered to the common input.
I measured voltages and found.
Va-Vb=120V
Va-Vc=120V
Vb-Vc=120V
Vb-Vcom=0V
Everything appeard to be corrct here but there may be a wiring issue I have to trace and match up with the Open Delta VT connection to the realy per the wiring diagram as dpc mentioned.
RE: Incorrect CT Phase Angles leading to wrong power readings
RE: Incorrect CT Phase Angles leading to wrong power readings
Start again.
Please open your PT connection drw. and meter manual.
As Dave said, you have check connection of PT at the manual and drw.
In the modern meters you have several options for the PT connection and it's selecteble in the setting of meter.
Good Luck.
Slava
RE: Incorrect CT Phase Angles leading to wrong power readings
How is going,have you already find out that wrong with multilin 750?
Have you already check CT,VT input of 750 and also from VT and CT to the relay input must be same phase(not wrong wiring).
jack
RE: Incorrect CT Phase Angles leading to wrong power readings
For the correct operation of many relay features, the instrument transformer polarities
shown above in the Typical Wiring Diagram on page 3–8 must be followed. Note the solid
square markings shown with all instrument transformer connections. When the
connections adhere to this drawing, the arrow shows the direction of power flow for
positive watts and the positive direction of lagging vars. The phase sequence is user
programmable to be either ABC or ACB rotation.
good luck
jack
RE: Incorrect CT Phase Angles leading to wrong power readings
It turns out I discovered that the reason the Vca phasor was reading 300deg instead of 240deg was because there is an error in the PQMII meter that calculates this phasor wrong.
There was a firmware revision to correct this issue that was no implemented on my meters. So it turned out that it wasn't a wiring problem but rather an issue with the meter itself.
Just thought I'd share this with everyone who helped.
Thanks Again!